Continuing report on the deterioration of x-ray fluoroscopic screens.

Abstract

At the annual meeting of this Society in 1967, a preliminary report was presented on the deterioration of x-ray fluoroscopic screens (1). This paper constitutes a report on findings since that time. Equipment and Methodology Twenty-eight fluoroscopic screens are being used in this study, 14 each from two manufacturers. All screens are of the zinc-cadmium sulfide phosphor type with the phosphor bonded to plastic backing material. The screens have been divided into test groups and are being exposed to x rays, light from sun lamps, incandescent lamps, and fluorescent lamps. Most of the conditions were designed to be related to some realistic exposure to which the screen in a diagnostic x-ray unit might be subjected. Although it is known that light output of fluoroscopic and radiographic screens is influenced by temperature, no attempt has been made to stabilize the temperature since a set of controls is used for comparison. In normal use temperature would be expected to vary and this should be kept in mind in assessing screen quality when no control is available. The light output of all the screens is measured with a special x-ray unit and a direct-reading light meter employing a photomultiplier tube as the detector. By use of a direct-reading instrument, calibrated in foot-lamberts, measurements are made independently of the sensitivity of the observer's eye and require no time-consuming matching of brightness. Calibration of the light meter is maintained by means of a regulated brightness source. When measurements of the luminescent response of the fluoroscopic screens are made, three sheets of one-half-inch-thick MIX-D, a tissue-equivalent phantom material, are placed at the table-top position. A fixed geometry has been used since the initiation of the study. All screens are covered by lead glass, having an equivalent of 2.0 mm of lead at 150 kVp, so that all light readings are taken under normal conditions of viewing. The light output of the screens is measured for an exposure rate of 0.5 R/min. when the x-ray unit is operated at 100 kVp. The grouping of the screens is shown in TABLE 1. Each of the test conditions listed was chosen to investigate one of the agents reported to cause or suspected of causing screen deterioration. The exposure of the screens to light from fluorescent, tungsten, and sun lamps is made through lead glass. An additional set of screens in the sun lamp group is being exposed without being covered by lead glass. All x-ray exposures are made with the screens covered by lead glass so that back-scatter conditions would be the same as those experienced by a fluoroscopic screen in a fluoroscopic screen assembly. Test Conditions3 A. Control Group: Screens kept in a darkened storage box at room temperature and exposed to x rays only when periodic measurements (approximately once a month) of all the screens are made.